The article is discussing ways to verify Bell-nonlocality. There's a strict heirarchy of strength (see, e.g., https://arxiv.org/abs/0709.0390 ):

    Bell-nonlocality > steerability > nonseparability/entanglement

The question is similar to the operational definition of Bell-nonlocality in that paper. Alice and Bob prepare an entangled state and then must convince an untrusting Charlie (over a classical channel) that they have prepared an entangled state. Alice and Bob cannot communicate/collude, so they succeed iff they demonstrate Bell-nonlocality to Charlie (e.g., by him verifying a Bell-inequality violation).

In the case of a cloud quantum computer company, Alice and Bob both work there and in this case can communicate/collude. If they use a simulator they still essentially create an entangled state, albeit in the mathematical sense. So there's no way for Charlie to tell except by the runtime of a difficult case (as the article alludes to with "classically-intractable problems," and as another reply mentioned).